In oil and gas wells, when carbonate reservoir rocks are acidized with hydrochloric acid, a few dominating channels or wormholes may be created, with a majority of the acid flow along these channels, leaving the rest of the reservoir rock unstimulated. This selective stimulation is undesirable, particularly when long vertical or horizontal sections are to be treated. To overcome these shortcomings, methods of diverting the treating fluid were developed to divert the flow of acid from areas of high permeability, to ensure the treatment of the entire section. Over the years, many chemicals including polymer gels, foam, oil-soluble resins and rock salt have been used as diverting agents. Field application of these diverting agents requires pumping of multiple stages of alternating acid and diverting agent.
More recently, self-diverting acids have been employed to simplify the process and to eliminate the need for multiple stages of alternating acid and diverting agent. The self-diverting acids employ the use of viscoelastic surfactants (VES). The viscoelastic surfactants are designed to gel at intermediate pH levels. When the acid treating fluid is initially at a low pH, the VES has a very low viscosity, making the VES acid treating fluid easy to pump and flow into the pores and channels of the formation. As the acid begins to react, the fluid undergoes an increase in viscosity due to the increases in calcium ions and pH, thus causing in-situ gelling of the acid. The higher viscosity of the gelled VES temporarily blocks the wormholes and channels formed in the formation, allowing the acid to divert to other untreated areas. The viscosity of the gelled acid can be completely reduced by the introduction of a mutual solvent or by produced hydrocarbon during flow-back.
In any of these acid treatments, the acids used come in contact with metals while pumping and during flowing back, which can damage the metal components and equipment used. As a result, the presence of suitable acid corrosion inhibitors are required for the acidizing treatments. Acid corrosion inhibitors (ACI) used for oilfield applications normally contain formic acid components or formic-acid-producing compounds when exposed to well conditions. Although these corrosion inhibitors have been successfully used in well stimulation operations, formic acid and other short-chained aliphatic acids and their related aldehydes or precursors typically present in corrosion inhibitors have been recently associated with the corrosion of pipelines and/or other equipment. Therefore, ACIs that are free of such acids or acid precursors are highly desirable for acid stimulation applications.